Occasionally, deactivating a WordPress plug-in can bring on the white screen of death. This generally occurs if the site or other plug-in are interdependent on each other. Obviously, when this occurs, the admin is unable to access the plug-in screen to activate the plugin. Hopefully, there is a backup with files AND database that you can restore.
If not, according to a Stack Exchange forum post, you must edit the WordPress database. Specifically, the row “active_plugins” on the “Options” table is the record that will need to be updated. Don’t forget, backup your database. No need to make things worse.
Read the solution on Stack Exchange – Activate a plugin through PHPMyAdmin.